Multimission and multispectral sonar

ABSTRACT

A survey system including a transmitter, receiver, projector array and hydrophone array transmits and receives sound waves to perform one or more survey missions.

PRIORITY APPLICATIONS AND INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.17/359,511 filed Jun. 26, 2021 which is a continuation of U.S. patentapplication Ser. No. 16/158,551 filed Oct. 12, 2018 which is a which isa continuation of U.S. patent application Ser. No. 15/476,137 filed Mar.31, 2017, now U.S. Pat. No. 10,132,924, which claims the benefit of U.S.Prov. Pat. App. 62/329,631 filed Apr. 29, 2016. This applicationincorporates by reference, in their entireties and for all purposes, thedisclosures of U.S. Pat. No. 3,144,631 concerning Mills Cross sonar,U.S. Pat. No. 8,305,841 concerning sonar used for mapping seafloortopography, U.S. Pat. No. 7,092,440 concerning spread spectrumcommunications techniques, U.S. Pat. No. 5,483,499 concerning Dopplerfrequency estimation, and U.S. Pat. No. 9,244,168 concerning frequencyburst sonar.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to underwater acoustical systems, methodsfor using underwater acoustical systems, and methods for processing andusing the data they produce. In particular, the invention relates tosurvey systems including sonar systems with methods of use that enablemultiple survey missions to be carried out simultaneously while using asingle array of transmitting transducers and a single array of receivingtransducers.

Discussion of the Related Art

A month after the Titanic struck an iceberg in 1912, Englishmeteorologist Lewis Richardson filed a patent at the British PatentOffice for an underwater ranging device. Modern day successors toRichardson's invention are often referred to as SONAR (sound navigationand ranging) devices. Among these devices are ones using transducerarrays to project sound or pressure waves through a liquid medium andtransducer arrays to receive corresponding echoes from features thatscatter and/or reflect impinging waves.

Information about these features and their environment can be derivedfrom the echoes. For example, bathymetric surveys provide informationabout the depth of scattering centers, water column surveys provideinformation about scattering centers in the water column, and seafloorcharacterization surveys provide information about scattering centers atthe seafloor surface and below the seafloor surface.

The diversity and quality of the information returned in echoes may bedetermined in part by the characteristics of the signal used to excitethe projector transducers. The cost of obtaining this information isstrongly influenced by the timeframe during which manpower and equipmentis required to acquire the information.

Although some progress towards improving data quality and diversitywhile reducing the time required to perform an underwater survey hasbeen made, particularly through the use of multibeam echo sounders, longstanding technological challenges and risks associated with building andtesting costly new survey equipment present significant obstacles tofurther similar improvements.

SUMMARY OF THE INVENTION

The present invention provides a survey system including a multibeamecho sounder and/or portions thereof. In an embodiment, the presentinvention provides a survey system for performing multiple missions permessage cycle, the survey system including a multibeam echo soundersystem for installation on a water going vehicle, the survey systemcomprising: an acoustic transceiver for use with one or more transducersin a single projector array and plural transducers in a singlehydrophone array; the projector array arranged with respect to thehydrophone array to form a Mills Cross; the transceiver for use with aplurality of N non-overlapping frequency bands having respectivebandwidths and center frequencies; the transceiver for synthesizing atransmitter message that incorporates one or more signals from each ofthe frequency bands, the signals supporting a plurality of missions;and, the message for exciting the projector array such that a swath of awaterbody bottom is ensonified by each of the signals in the message anda message echo from ensonified scattering centers is returned to thehydrophone array; wherein a first of the frequency bands is forsupporting a first mission and a second of the frequency bands is forsupporting a second mission, the first mission frequency band beingwidely spaced apart from the second mission frequency band to promotesurvey system recognition of one or more frequency dependentcharacteristics of the ensonified scattering centers. Notably, surveydata may be collected from ensonification of features in waterbodies ingeneral including any of oceans, seas, bays, fiords, estuaries, lakes,rivers, navigable waterways, canals, and harbors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingfigures. These figures, incorporated herein and forming part of thespecification, illustrate embodiments of the invention and, togetherwith the description, further serve to explain its principles enabling aperson skilled in the relevant art to make and use the invention.

FIG. 1A shows a survey system including a multibeam echo sounder systemof the present invention.

FIGS. 1B-E show embodiments of at least portions of the multibeam echosounder system of FIG. 1A.

FIG. 2A shows a message cycle for use with the multibeam echo soundersystem of FIG. 1A.

FIGS. 2B-E show messages including plural signals at differentfrequencies for use with the system multibeam echo sounder system ofFIG. 1A.

FIGS. 3A-D show swaths ensonified by multifrequency messages of themultibeam echo sounder of FIG. 1A.

FIG. 4 shows a table of survey missions to be performed using themultibeam echo sounder of FIG. 1A.

FIG. 5 shows a table of multimission surveys to be performed using themultibeam echo sounder of FIG. 1A.

FIGS. 6A-G show multimission surveys to be performed using the multibeamecho sounder of FIG. 1A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosure provided in the following pages describes examples ofsome embodiments of the invention. The designs, figures, and descriptionare non-limiting examples of the embodiments they disclose. For example,other embodiments of the disclosed device and/or method may or may notinclude the features described herein. Moreover, described features,advantages or benefits may apply to only certain embodiments of theinvention and should not be used to limit the disclosed invention.

As used herein, the term “coupled” includes direct and indirectconnections. Moreover, where first and second devices are coupled,intervening devices including active devices may be locatedtherebetween.

FIGS. 1A-E show a survey system including a multibeam echo soundersystem and describe multibeam echo sounder embodiments.

FIG. 1A shows a survey system in accordance with an embodiment of thepresent invention 100A. The survey system includes an echo soundersystem such as a multibeam echo sounder system 102 which may be mountedon a surface vehicle or vessel, a remotely operated vehicle, anautonomous underwater vehicle, or the like. As is further describedbelow, echo sounder and/or survey system outputs 114 may becontemporaneous with echo sounder processing of hydrophone data as insome embodiments for bathymetry or non-contemporaneous with processingof hydrophone data as in some embodiments for waterbody bottomclassification.

Data acquired by multibeam echo sounder systems 104 includes data fromecho sounder listening devices such as hydrophones (e.g., transducers)that receive echoes which are related to the acoustic/pressure wavesemanating from the echo sounder projectors but have returned by virtueof an interaction with inhomogeneities of many kinds. The interactionsmake take the form of reflection or scattering. The inhomogeneities,also known as reflectors and scattering centers, representdiscontinuities in the physical properties of the medium. Scatteringcenters may be found in one or more of i) an ensonified volume of thewaterbody such as a water column, upon the ensonified surface of thebottom, or within the ensonified volume of the sub-bottom.

Scattering centers of a biological nature may be present in the watercolumn, as they are a part of the marine life. Scattering centers of anonbiological nature may be present in the water column in the form ofbubbles, dust and sand particles, thermal microstructure, and turbulenceof natural or human origin, such as ships' wakes. Scattering centers onthe surface of the bottom may either be due to the mechanical roughnessof the bottom, such as ripples, or be due to the inherent size, shapeand physical arrangement of the bottom constitutes, such as mud, sand,shell fragments, cobbles and boulders, or due to both the two factors.Scattering centers in the sub-bottom may be due to bioturbation of thesediments, layering of different sediment materials within the bottom orburied manmade structures such as pipelines.

Data processing within the echo sounder system may includecontemporaneous processing of hydrophone data 106, for example to obtainbathymetric and/or backscatter data. Data processing may also includenon-contemporaneous processing of multibeam echo sounder system data108, for example to characterize bottom conditions or the water column.

Data processing may include utilization of complementary or other data.For example, contemporaneous processing of hydrophone data 106 mayutilize contemporaneous 110 and/or non-contemporaneous 112 data such ascontemporaneously collected geographic positioning system (“GPS”) data,sound speed measurements, attitude, and navigational information. Forexample, non-contemporaneous processing of echo sounder system data mayutilize contemporaneous 110 and/or non-contemporaneous 112 data such asnon-contemporaneously collected waterbody bottom composition data andtidal records.

FIG. 1B shows portions of a first multibeam echo sounder system (“MBES”)100B. The echo sounder system includes a transducer section 120 and anacoustic transceiver 122. The echo sounder system may include atransceiver interface such as an interface module 124 and/or aworkstation computer 126 for one or more of data processing, datastorage, and interfacing man and machine. Here, transducers in a MillsCross arrangement 120 include a transmitter or projector array 130 and areceiver or hydrophone array 140. Projectors in the projector array maybe spaced along a line that is parallel with a keel line or track of avehicle to which they are mounted which may be referred to as an alongtrack arrangement. In some embodiments, a receiver of the transceiver122 has an operating frequency ranged matched with that of theprojectors and/or the hydrophones.

During echo sounder operation, sound or pressure waves emanating fromthe projector array travel within a body of water and possibly withinthe bottom beneath the body of water and in doing so may undergointeractions such as reflections or scattering, which disturb thepropagation trajectory of the pressure waves. Some of the reflections orechoes are “heard” by the hydrophone array. See for example thedisclosure of Etal, U.S. Pat. No. 3,144,631, which is included herein byreference, in its entirety and for all purposes.

The acoustic transceiver 122 includes a transmitter section 150 and areceiver section 170. The acoustic transceiver may be configured totransmit to a single projector array 130 and to receive from a singlehydrophone array 140. In some embodiments, such a transceiver may besaid to operate with a single transmitter array and a single receiverarray. Unless otherwise noted, the term transceiver does not requirecommon transmitter and receiver packaging.

The echo sounder may further include an interface module such as aninterface module 124 for interconnection with the transceiver 122. Thisinterface module may provide, among other things, a power supply for thetransceiver, communications with the transceiver, communications withthe workstation computer 126, and communications with other sources ofdata such as a source of contemporaneous GPS data.

The workstation computer 126 may provide for one or more of dataprocessing such as data processing for visualization of survey results,for data storage such as storage of bathymetry data and backscatterdata, for user inputs, and for display of any of inputs, system status,and survey results.

FIG. 1C shows portions of a second multibeam echo sounder system(“MBES”) 100C. The echo sounder system includes a transducer section120, a transmitter section 150, and a receiver section 170. Someembodiments include an interface section 190 and/or a management section192.

The transducer section includes transducers for generating acousticmessages and transducers for receiving acoustic messages. For example, atransducer section may include an array of projectors 130 and an arrayof hydrophones 140.

Projectors in the projector array may include piezoelectric elementssuch as ceramic elements which may be stacked or not. Element geometriesmay include circular and non-circular geometries such as rectangulargeometries. Some projectors have an operating frequency range of about10 kHz to 100 kHz, of about 50 kHz to 550 kHz, or about 100 to 1000 kHz.

Hydrophones in the hydrophone array may include piezoelectric elementssuch as ceramic elements. Element geometries may include circular andnon-circular geometries such as rectangular geometries. Some hydrophoneshave an operating frequency range of about 10 kHz to 100 kHz, of about50 kHz to 550 kHz, or about 100 to 1000 kHz.

During operation of the projector array 130 and hydrophone array 140,transmitter section excites the projector array, an outgoing message 137emanates from the projector array, travels in a liquid medium to areflector or scattering center 138, is reflected or scattered, afterwhich a return or incoming message 139 travels to the hydrophone array140 for processing by the receiver 170. Notably, the acoustic/pressurewave input 136 received at the hydrophone array 140 may include aperturbed version of the transmitted message 137 along with spurioussignal and/or noise content.

The transmit section 150 may include a signal generator block 158, atransmit beamformer block 156, a summation block 154, and a poweramplifier block 152. The transmit section provides for generation ofsignals 158 that will be used to compose the message 137. Notably, amessage may be composed of multiple signals or not. Where a message iscomposed of multiple signals, the message may contain i) signals inparallel (superposed), ii) signals that are serialized (concatenated),or may be a combination of parallel and serial signals. In anembodiment, plural signals are generated and transmitted at pluraldifferent center frequencies S_(cf1), S_(cf2) . . . .

The transmit beamformer block 156 receives the signal(s) from the signalgenerator block 158 where beamforming for each signal takes place. Thebeam(s) are combined in the summation block 154 to construct a parallel,serial, or combination message M. In the power amplifier block 152, thetime series voltages of the message are amplified in order to excite ordrive the transducers in the projector array 130. In an embodiment, eachtransducer is driven by a respective amplifier.

The receive section 170 includes multiple hydrophone signal processingpipelines. In an embodiment the receive section includes a hardwarepipelines block/analog signal processing block 172, a software pipelinesblock/digital signal processing block 174, a receive beamformer block176 and a processor block 178. The receive section provides forisolating and processing the message 137 from the input 136 received atthe hydrophone array 140. For example, some embodiments process echoesto determine depths as a function of, among other things, round triptravel times that are based on matching a transmitted message 137 with acorresponding received message isolated from the hydrophone array input136.

In the hardware pipeline block 172, plural hydrophone array transducersof the hydrophone array 140 provide inputs to plural hardware pipelinesthat perform signal conditioning and analog-to-digital conversion. Insome embodiments, the analog-to-digital conversion is configured foroversampling where the converter Fin (highest input frequency) is lessthan F_(s)/2 (one half of the converter sampling frequency). In anembodiment, a transceiver 122 operating with a maximum frequency ofabout 800 kHz utilizes analog-to-digital converters with sampling ratesof five MHz.

In the software pipeline block 174, the hardware pipelines 172 provideinputs to the software pipelines. One or more pipelines serve each ofthe hydrophones in the hydrophone array. Each pipeline providesdownconversion and filtering. In various embodiments, the filterprovides for recovery of a message from a hydrophone input 136. In anembodiment, each hydrophone is served by plural pipelines fordeconstructing a multifrequency message into plural signals atrespective center frequencies S′_(cf1), S′_(cf2) . . . .

In the receive beamforming or steering block 176, the software pipelines174 provide beamformer inputs. Beamformer functionality includes phaseshifting and/or time delay and summation for multiple input signals. Inan embodiment, a beamformer is provided for each frequency S′_(cf1),S′_(cf2) . . . . For example, where software pipelines operate at twofrequencies, inputs to a first beamformer are software pipelinesoperating at the first frequency and inputs to a second beamformer aresoftware pipelines operating at the second frequency.

In the processor block 178, the beamformers of the beamformer block 176provide processor inputs. Processor functionality includes bottomdetection, backscatter processing, data reduction, Doppler processing,acoustic imaging, and generation of a short time series of backscattersometimes referred to as “snippets.”

In an embodiment, a management section 192 and a sensor interfacesection 190 are provided. The management section includes an interfacemodule 194 and/or a workstation computer 196. The sensor interfacesection provides for interfacing signals from one or more sensors ES1,ES2, ES3 such as sensors for time (e.g. GPS), motion, attitude, andsound speed.

In various embodiments, control and/or control related signals areexchanged between the management section 192 and one or more of thepower amplifier block 152, software pipelines block 174, transmitbeamformer block 156, receive beamformer block 176, signal generatorblock 158, processor block 178. And, in various embodiments sensorinterface section data 190 is exchanged with the management section 192and the processor block 178.

FIG. 1D shows portions of a second multibeam echo sounder system(“MBES”) 100D. The echo sounder system includes a transducer section120, a transmitter section 150, and a receiver section 170. Someembodiments include an interface section 190 and/or a management section192.

In the embodiment shown, a message 153 incorporating quantity N signalsat N respective different center frequencies is used to excite pluralprojectors in a projector array and a receiver having quantity Thardware or software pipelines and (T×N) hardware or software pipelinesmay be used to process T hydrophone signals for recovery of echoinformation specific to each of the N frequencies.

The transmitter section 150 is for exciting the projector array 130. Thesection includes a signal generator block 158, a transmit beamformerblock 156, a summation block 154, and a power amplifier block 152.

The signal generator block 158 generates quantity N signals S1, S2 . . .SN. The signals have center frequencies cf₁, cf₂ . . . cf_(n) which maybe spaced at intervals of, for example, 50 to 150 kHz. In an embodimentthe signals are spaced at intervals of at least 100 kHz.

A transmit beamformer block 156 receives N signal generator blockoutputs. For each of the N signals generated, the beamformer blockproduces a group of output beam signals such that there N groups ofoutput beam signals.

The summation block 154 receives and sums the signals in the N groups ofoutput beams to provide a summed output 153.

The power amplifier block 152 includes quantity S amplifiers for drivingrespective projectors in the projector array 130. Each power amplifierreceives the summed output 153, amplifies the signal, and drives arespective projector with the amplified signal.

An array of quantity T hydrophones 140 is for receiving echoes ofacoustic/pressure waves originating from the projector array 130. Theresulting hydrophone signals are processed in the receiver section 170which includes a hardware pipeline block 172, a software pipeline block174, a receive beamformer block 176, and a processor block 178.

In the hardware pipeline block 172, T pipelines provide independentsignal conditioning and analog-to-digital conversion for each of the Thydrophone signals.

In the software pipeline block 174, (T×N) software pipelines providedownconversion and filtering at N frequencies for each of the T hardwarepipeline outputs. As shown, each of T hardware pipeline outputs 181,182, 183 provides N software pipeline inputs a,b and c,d and e,f (i.e.,3×2=6 where T=3 and N=2).

In the receive beamformer block 176, (T×N) software pipeline block 174outputs are used to form N groups of beams. A beamformer is provided foreach of the N frequencies. For example, where there are T=3 hydrophonesand software pipelines operate at N=2 frequencies, inputs to a firstbeamformer are software pipelines operating at the first frequency a₁,c₁, e₁ and inputs to a second beamformer are software pipelinesoperating at the second frequency b₁, d₁, f₁.

In the processor block 178, N processors receive respective groups ofbeams formed by the beamformer block 176. Processor block 178 data isexchanged with a management section 192 and sensor interface 190 dataES1, ES2, ES3 is provided to the management section and/or the processorblock.

In various embodiments control signals from the management block 192 areused to make power amplifier block 152 settings (e.g., for “S” poweramplifiers for shading), to control transmit 156 and receive 176beamformers, to select software pipeline block 174 operatingfrequencies, and to set signal generator block 158 operatingfrequencies.

As the above illustrates, the disclosed echo sounder transmitter mayconstruct a message incorporating signals at N frequencies. And, theecho sounder may utilize a receiver having T hardware pipelines and(T×N) software pipelines to process T hydrophone signals for recovery ofecho information specific to each of the N frequencies.

FIG. 1E shows portions of a third multibeam echo sounder system (“MBES”)100E. The echo sounder system includes a transducer section 120, atransmitter section 150, and a receiver section 170. Some embodimentsinclude an interface section 190 and/or a management section 192.

In the embodiment shown, a message 153 incorporating first and secondsignals S_(cf1), S_(cf2) at first and second different centerfrequencies N=2 is used to excite three projectors in a projector array,and a receiver having three hardware pipelines and six softwarepipelines is used to process three hydrophone signals T=3 for recoveryof echo information specific to each of the N frequencies.

The transmitter section 150 is for exciting the projector array 130. Thesection includes a signal generator block 158, a transmit beamformerblock, a summation block 154, and a power amplifier block 152.

In the signal generator block 158, N=2 signal generators are shownoperating different user selectable center frequencies f1, f2. Inrespective beamformers of the beamformer block 156, multiple beams aregenerated from each signal. In a summation block 154, the beams arecombined to produce a summation block output signal 153.

The transducer block 120 includes a projector array 130 and a hydrophonearray 140 arranged as a Mills Cross. As shown, there are threeprojectors 131 in the projector array and three hydrophones 141 in thehydrophone array. In the power amplifier block 152, the summed signal ormessage 153 is an input to power amplifiers driving respectiveprojectors.

Applicant notes that for convenience of illustration, the projector andhydrophone counts are limited to three. As skilled artisans willappreciate, Mills Cross arrays need not have equal numbers of projectorsand hydrophones nor do the quantities of either of these transducersneed to be limited to three. For example, a modern multibeam echosounder might utilize 1 to 96 or more projectors and 64 to 256 or morehydrophones.

The array of T=3 hydrophones 141 is for receiving echoes resulting fromthe acoustic/pressure waves originating from the projector array 130.The resulting hydrophone signals are processed in the receiver section170 which includes a hardware pipeline block 172, a software pipelineblock 174, a receive beamformer block 176, and a processor block 178.

In the hardware pipelines block 172, each of T=3 hardware pipelinesprocesses a respective hydrophone 141 signal through analog componentsincluding an analog-to-digital converter. In the embodiment shown, ahardware pipeline provides sequential signal processing through a firstamplifier, an anti-aliasing filter such as a low pass anti-aliasingfilter, a second amplifier, and an analog-to-digital converter.

In the software pipelines block 174, each of the T=3 hardware pipelineoutputs is processed through N=2 software pipelines with downconversionand matched filtering. In the embodiment shown, a software pipelineprovides sequential signal processing through a mixer (oscillator is notshown for clarity), a bandpass filter, a decimator, and a matchedfilter. Communications may occur via communications links between any ofthe processor block 178, the signal generator block 158, the hardwarepipelines block 172, the software pipelines block 174, the and thebeamformer block 176. See for example FIGS. 1C-D.

In the receive beamformer block 176, each of N=2 beamformers processessignals. As such, three software pipeline outputs at a first centerfrequency are processed by a first beamformer and three softwarepipeline outputs at a second center frequency are processed by a secondbeamformer. Notably, beamformers may be implemented in hardware orsoftware. For example, multiple beamformers may be implemented in one ormore field programmable gate arrays (“FPGA”).

In the processor block 178, each of N=2 processors are for processingrespective beamformer outputs. Here, a first plurality of beamsgenerated by the first beamformer is processed in a first processor anda second plurality of beams generated by the second beamformer isprocessed in a second beamformer. Processor outputs interconnect with amanagement section 192. Notably, one or more processors may beimplemented in a single device such as a single digital signal processor(“DSP”) or in multiple devices such as multiple digital signalprocessors.

Complementary data may be provided via a sensor interface section 190that is interfaced with a plurality of sensors ES1, ES2, ES3. The sensorinterface module may provide sensor data to the management section 192and/or to processors in the processor block 178.

The management section 192 includes a sonar interface 194 and/or aworkstation computer 196. In various embodiments control signals fromthe management block 192 are used for one or more of making poweramplifier block 152 settings (e.g., for array shading), controllingtransmit 156 and receive 176 beamformers, selecting software pipelineblock 174 operating frequencies, setting set signal generator block 158operating frequencies, and providing processor block 178 operatinginstructions.

Applicant notes that the echo sounder systems of FIGS. 1C-E may be usedto process hydrophone returns from targets i) present within anensonified volume of the water body, ii) upon an ensonified surface ofthe bottom, or iii) lying within an ensonified volume of the bottom.

FIGS. 2A-E show a ping cycle and describe multifrequency messagestransmitted in the ping cycle.

FIG. 2A shows a message cycle 200A. The cycle includes a sequence ofoperations with transmission of a message during a time t1 and receptionof a message during a time t3. Transmission of a message refers to theprocess that excites the projector array 130 and reception of a messagerefers to the complementary process that interprets the message echoreceived by the hydrophone array 140. A wait time t2 that variesprimarily with range, angle, and sound speed may be interposed betweenthe end of the message transmission and the beginning of the messagereception. This wait time may be determined by round trip travel timefor the longest sounding range, for example a return from the mostdistant cell in a swath ensonified by the projector array. In someembodiments, the message transmit length is in a range of 10 to 60microseconds. In some embodiments, the transmit message length is about10 milliseconds.

FIG. 2B shows a first multifrequency message 200B. In the embodimentshown, the message includes three signals which may comprise a single ormultiple waveforms. A first signal occupies at least a portion of arelatively lower frequency band, for example a band extending from 100kHz to 200 kHz. A second signal occupies at least a portion of anintermediate band, for example a band extending from 300 kHz to 400 kHz.A third signal occupies at least a portion of a higher band, for examplea band extending from 500 kHz to 600 kHz. This message may be referredto as a multifrequency message with signals in widely spaced frequencybands. The frequency bands used by the signals may be referred to as thesignal bands wherein a gap band exists between a first signal band and anearest neighboring second signal band.

Widely spaced frequency bands are contrasted with narrowly spaced andminimally spaced frequency bands. In practice, such frequency bands maybe spaced apart just enough to enable separation of signals and/or toprevent interference of the signals. Where signal separation relies onbandpass filters, bandpass filter performance can determine the minimumspacing of frequency bands. For sonar systems operating in the 100 to400 kHz range, frequency band spacing may be about 1 to 3 kHz, about 3to 5 kHz, or about 5 to 10 kHz.

Where the same target is ensonified by signals in narrowly spacedfrequency bands, for example CW signals, the characteristics ofbackscatter from a signal in the first band are quite similar to thecharacteristics of backscatter from a signal in the second band. Here,differences in backscatter characteristics, for example backscatterstrength, may be small and/or beyond a range of detection.

Minimizing differences in backscatter characteristics is a desirableresult where the surveyor's objective is to increase the along tracksounding density. It is also a desirable result where the surveyor'sobjective is to utilize similar signal ranges and similar signalbackscatter strengths to simplify a process of normalizing survey data.However, signals in widely spaced frequency bands do not satisfy theseobjectives.

Unlike signals in narrowly spaced frequency bands, signals in widelyspaced bands may be chosen to elucidate what frequency dependentdifferences a backscatterer or echo source (e.g., a waterbody bottom)presents.

Further, where backscatterer response is frequency dependent, projectoremissions in widely spaced frequency bands may be used to elicitdistinguishable backscatter responses. This may be the case even whenthe echoes are returned from the same scattering areas.

In an embodiment, two frequency bands including respective signals maybe widely spaced when they are not narrowly spaced. In an embodiment,two frequency bands including respective signals are widely spaced whenthey are not narrowly spaced.

In an embodiment, two frequency bands including respective signals arewidely spaced when their center frequencies are separated by at leasttwice the bandwidth of the narrowest of the signals. And, in anembodiment, two frequency bands including respective signals are widelyspaced when their center frequencies are separated by at least 30% ofthe lower of the two center frequencies. In an embodiment, two frequencybands including respective signals are widely spaced when both of theseconditions are met.

In an embodiment, two frequency bands including respective signals arewidely spaced when a statistically significant difference exists betweena message echo portion attributable to the first signal and a messageecho portion attributable to the second signal. Echo characteristicswhich may be evaluated for statistically significant differences includemean backscatter strength, angular response of backscatter strength, andmaximum detection range.

In an embodiment, two frequency bands including respective signals arewidely spaced when the strength of essentially cotemporaneousbackscatter from the signals differs by a prescribed amount. Notably,backscatter strength often increases with frequency while increasing theangle of incidence reduces backscatter strength. In some embodiments,angle average backscatter strength may be compared to indicate whetherthe frequencies are widely spaced. In some embodiments, backscatterstrengths at a particular angle of incidence θ such as projector angleof incidence are the basis of the above comparison and a difference ofabout 2 dB or more may indicate that the frequencies are widely spaced.An example follows.

Consider a message comprising signals Sx and Sy in frequency bands Bxand By where Bx is the lower of the two frequency bands. Wherebackscatter signal strengths BSx and BSy at a common angle of incidenceθ are such that BSy exceeds BSx by 2 dB or more, then the frequencybands are widely separated. In an embodiment, the length of the messageis less than about ten milliseconds.

Any one or more of the above described methods may be used to determinewhether frequency bands are widely spaced.

As discussed below, the number of signals within a message and thespacing of corresponding frequency bands may be varied to suitparticular applications and environmental conditions.

FIG. 2C shows a second multifrequency message 200C. In the embodimentshown, the message includes three signals which may abut in time,substantially abut in time (e.g., gap of less than about ten percent ofone of, or of the shortest of, the signal lengths), or be spaced apartin time (e.g., a gap of about ten percent or more of one of, or of theshortest of, the signal lengths). A first signal 232 in a relatively lowfrequency band and beginning at time t_(s1i) is followed in time by asecond signal 234 in an intermediate frequency band. The second signalis followed in time by a third signal 236 in a high frequency bandending at time t_(s3ii). The figure illustrates a multifrequency messagewith serial or serialized signals.

Unlike FIG. 2C, FIG. 2D shows a multifrequency message that is deliveredin multiple pings 200D. In the embodiment shown, the message includesthree signals which are not sent in a single ping message but rather ina multi-ping message using three consecutive message cycles. Thesemessage cycles may abut in time, substantially abut in time (e.g., gapof less than about ten percent of one of, or of the shortest of, thesignal lengths), or be spaced apart in time (as shown, e.g., a gap ofabout ten percent or more of one of, or of the shortest of, the signallengths). A first signal (Signal 1) in a relatively low frequency bandand between times tu1 and tu2 corresponding to a first ping. A secondsignal (Signal 2) in an intermediate frequency band and between timestu3 and tu4 corresponding to a second ping. A third signal (Signal 3) ina relatively high frequency band and between times tu5 and tu6corresponding to a third ping.

FIG. 2E shows a third multifrequency message 200E. In the embodimentshown, the message includes three signals that overlap in time with theearliest signal beginning at t_(a) and the latest signal ending att_(b). Here, a first signal 242 in a relatively lower frequency band, asecond signal 244 in an intermediate frequency band, and a third signal246 in a high frequency band illustrate a multifrequency message withparallel signals.

FIG. 3A shows ensonification of a waterbody bottom by a multifrequencymessage with serialized signals 300A. Along a track of a multibeam echosounder vehicle, three swaths are ensonified by the serial messageincorporating three signals. Here, a low band swath is ensonified by afirst signal in a lower frequency band, an intermediate band swath isensonified by a second signal in an intermediate frequency band, and athird high band swath is ensonified by a third signal in a high bandswath. The swaths are displaced along the track due to signals emittedsequentially in time and movement of the sonar with time along thesurvey track. Notably, the lower band swath has the greatest width w1while the higher band swath has the least width w3 (w1>w2 and w2>w3) dueto the (1/frequency²) relationship between signal frequency and range.Further, swaths associated with higher bands may be narrower in thealong-track direction than the swaths of lower bands.

FIG. 3B shows ensonification of a waterbody bottom by a multifrequencymessage with parallel signals 300B. Along a track of a multibeam echosounder platform, three swaths are ensonified by the messageincorporating three signals in parallel. Here, a lower band swath isensonified by a first signal in a lower frequency band, an intermediateband swath is ensonified by a second signal in an intermediate frequencyband, and a third high band swath is ensonified by a third signal in ahigher band swath. Because the message incorporates parallel signals,the swaths are not displaced along the track. Rather, the swaths arespatially superposed because the parallel signals are superimposed(overlap) in time.

FIG. 3C illustrates returns from a section of the bottom of the waterbody that is shared by multiple swaths ensonified by a multifrequencymessage with parallel signals 300C. Here, the parallel signals of FIG.3B ensonify lower, intermediate, and higher band swaths along respectiveswath widths w1, w2, w3. Because the parallel signal message avoidsswath displacement due to vehicle motion, a swath area common to all ofthe swaths provides common echo generations for each of the signals.This common area is within an area 322 of the higher band swath.

FIG. 3D illustrates colocated swaths ensonified by a multifrequencymessage with parallel signals 300D. Here, each of lower, intermediate,and higher band swaths share a common width w. Various configurationsmay produce colocated swaths.

In a first example, swaths may be colocated by selecting echo sounderreceive beamforming 176 to a swath or sector width less than or equal tothe higher frequency band swath width. In a second example, swaths maybe colocated when grating lobes apparent at higher frequencies limitusable steering angles. In a third example, swaths may be colocated whenoperating in shallow water such that higher attenuation at higherfrequencies does not limit detection range. The beamwidths associatedwith each frequency band typically vary with frequency with higherfrequencies providing higher angular resolution. However, frequencydependence may be mitigated by normalizing to a common beamwidth bydisabling selected array elements to vary apertures of the transmit andreceive arrays.

FIG. 4 shows signal types and/or waveforms for use with various missionsand/or applications of a multibeam echo sounder 400. As seen in thetable, signal types include CW (continuous wave), FM (frequencymodulation), OSS (orthogonal spread spectrum), PC (phase-coded), PT(pulse train), and LPI (low probability of intercept). Notably, FMincludes linear FM or LFM.

The bathymetry and the forward-looking missions may use any of CW, FM,OSS, PC, PT, and LPI. The imaging mission may use any of CW, FM, OSS,and PC while the sub-bottom profiling mission may use any of CW, FM, PC,and PT. More selective missions are the water column mission that mayuse CW, FM, OSS, or PC, the bottom classification mission that may useCW or FM, and the Doppler mission that may use CW or PC.

Notably, as indicated above, a survey operation need not be limited to asingle mission or application as a survey vehicle progresses along atrack. Rather, various ones of the embodiments of the multibeam echosounder of the present invention utilizing a single projector array anda single hydrophone array may be used to acquire and to process data formultiple missions simultaneously. The number of substantiallysimultaneous missions that may be carried out as a survey vehicleprogresses along a track may equal or exceed the number ofnon-overlapping signal frequency bands accommodated by the multibeamecho sounder system 102.

The multimission surveys described below may utilize a multibeam echosounder system 102, 100B-E having a single projector array and a singlehydrophone array to substantially simultaneously acquire multimissionsurvey data as a survey vehicle progresses along a track (e.g. singlepass survey data). In some embodiments, substantially simultaneouslytakes into consideration motion of the survey vehicle and/or serialtransmit message signals.

FIG. 5 shows multimission message content and message construction tablefor exemplary multimission surveys 500. As seen in the table, amultimission survey is carried out using a multimission message whichmay be constructed in a particular manner.

A first multimission survey includes a first bathymetry mission and asecond bathymetry mission. Typically, intermediate bands are not used.

The first bathymetry mission utilizes a relatively low frequency bandwith a CW or FM signal. The second bathymetry mission utilizes arelatively high frequency band with a CW or FM signal. These signals maybe serialized or paralleled in a single ping message. These signals maybe sent in respective pings as a multi-ping message. Having readapplicant's disclosure, skilled artisans will recognize the advantagesof this multimission survey which, among other things, resolves longstanding problems associated with choosing one or the other of a highfrequency (relatively high resolution/relatively short range) survey ora low frequency (low resolution/long range) survey. In an embodiment,the frequency bands are widely spaced with band gaps therebetween.

A second multimission survey includes a first waterbody bottom orseafloor characterization mission and a second waterbody bottom orseafloor characterization mission. Typically, intermediate bands may beused.

The first waterbody bottom mission utilizes a relatively low frequencyband with a CW signal. The second waterbody bottom mission utilizes arelatively high frequency band with a CW signal. These signals may beparalleled in a single ping message. These signals may be sent inrespective pings in a multi-ping message. Having read applicant'sdisclosure, skilled artisans will recognize the advantages of thismultimission survey which, among other things, resolves long standingproblems associated with obtaining survey data sufficient for use insegmenting and/or classifying a waterbody bottom surface and/orwaterbody bottom subsurface where the echo response varies with sonarfrequency. Notably beneficial to bottom segmentation and/or bottomclassification survey missions are parallel signals in a single pingmessage that provide for echoes at multiple frequencies from the samebackscatterers (see e.g. FIGS. 3B-3D).

A third multimission survey includes a first waterbody bottomcharacterization or segmentation mission and a second bathymetrymission. Typically, intermediate bands may be used.

The first waterbody bottom characterization or segmentation missionutilizes a relatively lower frequency band with a CW signal or, in someembodiments, two or three CW signals. The second bathymetric missionutilizes a relatively higher frequency band with an FM signal. Thesesignals may be serialized or paralleled in a single ping. These signalsmay be sent in respective pings in a multi-ping message. Having readapplicant's disclosure, skilled artisans will recognize the advantagesof this multimission survey which, among other things, resolves longstanding problems associated obtaining survey data useful for bothcharacterization or segmentation of the waterbody bottom and bathymetryin a single pass.

A fourth multimission survey includes a first Doppler navigation missionand a second multi-fan bathymetric mission. Typically, no intermediatebands are used. Multi-fan may refer to plural quasi-parallel fans orswaths including a first fan and one or more additional fans steeredfore and/or aft of the first fan. For example, a multi-fan mission mightuse a central athwartship fan and quasi-parallel fans to either side ofthe athwartship fan.

The first Doppler navigation mission utilizes a relatively lowerfrequency band with a phase coded signal such as a Barker code. Thesecond multi-fan bathymetric mission utilizes a relatively highfrequency band with a spread spectrum signal such as orthogonal codedpulses OCP. These signals may be serialized in a single ping. BecauseOCP signals are distinguished by their code pattern, multiple ones ofthese coded signals may be used to ensonify respective parallel orsomewhat parallel swaths in a fan-like arrangement. The returns from theOCP signals are distinguished using the code patterns. These signals maybe serialized in a single ping or sent in respective pings in amulti-ping message. Having read applicant's disclosure, skilled artisanswill recognize the advantages of this multimission survey which, amongother things, resolves long standing problems associated with alongtrack sounding density, multi-aspect multibeam surveys, and concurrentbathymetric and navigation operations.

A fifth multimission survey includes a first sub-bottom profilingmission and a second bathymetry mission. Typically, intermediate bandsmay be used.

The first waterbody bottom mission utilizes a relatively low frequencyband with a CW signal. The second waterbody bottom mission utilizes arelatively high frequency band with a CW signal. These signals may beparalleled in a single ping message. These signals may be sent inrespective pings in a multi-ping message. Having read applicant'sdisclosure, skilled artisans will recognize the advantages of thismultimission survey which, among other things, resolves long standingproblems associated with obtaining survey data sufficient for use insub-bottom profiling and bathymetry. Notably beneficial to sub-bottomprofiling is parallel transmission of both the sub-bottom profilingsignal(s) and the bathymetry signal(s) such that the signals arereturned from the same backscatterers (see e.g. FIGS. 3B-3D).

A sixth multimission survey includes a first water columncharacterization mission and a second water column characterizationmission. Typically, intermediate bands may be used.

The first water column mission utilizes a relatively low frequency bandwith a CW or FM signal. The second water column mission utilizes arelatively high frequency band with a CW or FM signal. These signals maybe serialized or paralleled in a single ping message. These signals maybe sent in respective pings in a multi-ping message. Having readapplicant's disclosure, skilled artisans will recognize the advantagesof this multimission survey which, among other things, resolves longstanding problems associated with obtaining water column data sufficientfor use in segmenting and/or classifying water column scatterers wherethe echo response varies with sonar frequency. Notably beneficial towater column segmentation and/or water column classification missionsare parallel signals in a single ping message that provide for echoes atmultiple frequencies from the same backscatterers.

A seventh multimission survey includes a first water columncharacterization or segmentation mission and a second bathymetrymission. Typically, intermediate bands may be used.

The first water column characterization or segmentation mission utilizesa relatively lower frequency band with a CW or FM signal or, in someembodiments, two or three CW or FM signals. The second bathymetricmission utilizes a relatively higher frequency band with an FM signal.These signals may be serialized or paralleled in a single ping. Thesesignals may be sent in respective pings in a multi-ping message. Havingread applicant's disclosure, skilled artisans will recognize theadvantages of this multimission survey which, among other things,resolves long standing problems associated obtaining survey data usefulfor both characterization or segmentation of the water column andbathymetry in a single pass.

FIGS. 6A-G show exemplary messages with particular signal frequenciesfor use in multimission surveys 600A-G.

FIG. 6A shows a first multimission survey including a first long rangebathymetry mission and a second high resolution bathymetry mission 600A.

The first long range bathymetry mission utilizes a relatively lowerfrequency band with a CW or FM signal having a center frequency of about200 kHz and respective bandwidths of about 5 to 30 kHz and about 30 to60 kHz.

The second high resolution bathymetry mission utilizes a relativelyhigher frequency band with a CW or FM signal having a center frequencyof about 700 kHz and respective bandwidths of about 20 to 60 kHz andabout 20 to 60 kHz. These signals may be paralleled (as shown) in asingle ping message. These signals may be sent in respective pings in amulti-ping message.

FIG. 6B shows a second multimission survey including three bottomcharacterization or segmentation missions 600B.

The first bottom characterization or segmentation mission utilizes arelatively low frequency band with a CW signal having a center frequencyof about 50 kHz and a bandwidth of about 2 to 10 kHz.

The second bottom characterization or segmentation mission utilizes anintermediate frequency band with a CW signal having a center frequencyof about 100 kHz and a bandwidth of about 2 to 10 kHz.

The third bottom characterization or segmentation mission utilizes arelatively higher frequency band with a CW signal having a centerfrequency of about 150 kHz and a bandwidth of about 2 to 10 kHz. Thesesignals may be paralleled (as shown) in a single ping message. Thesesignals may be sent in respective pings in a multi-ping message.

These center frequencies at 50, 100, 150 kHz may be shifted to avoidharmonics. For example, where the 50 kHz center frequency locates thecenter of a first frequency band, first harmonics may be avoided byshifting the 50 kHz center frequency by a frequency incrementapproximating the width of the first frequency band. For example, wherethe 150 kHz center frequency locates the center of a second frequencyband, second harmonics may be avoided by shifting the 150 kHz centerfrequency by a frequency increment approximating the width of the secondfrequency band. As skilled artisans will understand, yet other similarchanges to the above center frequencies may avoid harmonics.

FIG. 6C shows a third multimission survey including a first bottomcharacterization or segmentation mission and a second bathymetry mission600C.

The first bottom characterization or segmentation mission utilizes arelatively low frequency band with three CW signals having respectivecenter frequencies of about 50, 150, 250 kHz. As described above, thesecenter frequencies may be shifted to avoid harmonics. And where, ashere, plural signals in respective bands are used to fulfill a singlemission, the mission may be referred to as a multiband mission.

The second bathymetric mission utilizes a relatively higher frequencyband with an FM signal having a center frequency of about 400 kHz and abandwidth of about 30 to 60 kHz. These signals may be serialized orparalleled (as shown) in a single ping message. These signals may besent in respective pings in a multi-ping message. Notably, the phrase“about . . . kHz” refers to manufacturing and operating tolerancesassociated with generation, transmission, reception, and/ordeconstruction of signals by modern day sonar equipment used forbathymetry and/or bottom segmentation.

FIG. 6D shows a fourth multimission survey including a first navigationmission and a second bathymetry mission 600D.

The first navigation mission utilizes a relatively lower frequency bandwith a phase coded signal having a center frequency of about 100 kHz anda bandwidth of about 60 kHz.

The second bathymetry mission utilizes a relatively higher frequencyband with three OSS signals having a center frequency of 400 kHz. TheOSS signals may have similar bandwidths and occupy a common band havinga bandwidth of about 100 kHz. Where, as here, there are multiple OSSsignals occupying a common band, this may be referred to as amultisignal band and the signals within this band may be referred to asa package of signals.

These signals may be sent in a message having a combination parallel andserial format with the bathymetry mission signals sent in parallel andthe navigation signal sent before or after the bathymetry signals.

FIG. 6E shows a fifth multimission survey including a first sub-bottomprofiling mission and a second bathymetry mission 600E.

The first sub-bottom profiling mission utilizes a relatively lowfrequency band with a CW signal having a center frequency of in a rangeof about 10 to 30 kHz, here 15 kHz, and a bandwidth of about 1 kHz.

The second bathymetry mission utilizes a relatively high frequency bandwith a CW signal having a center frequency of about 200 kHz and abandwidth of about 20 to 60 kHz. These signals may be paralleled (asshown) in a single ping message. These signals may be sent in respectivepings in a multi-ping message.

FIG. 6F shows a sixth multimission survey including a first water columnmission and a second water column mission 600F.

The first water column mission utilizes a relatively low frequency bandwith a CW or FM signal having a center frequency of about 100 kHz andrespective bandwidths of about 10 to 20 kHz and about 10 to 30 kHz.

The second water column mission utilizes a relatively higher frequencyband with a CW or FM signal having a center frequency of about 150 kHzand respective bandwidths of about 10 to 20 kHz and about 10 to 30 kHz.These signals may be paralleled (as shown) in a single ping message.These signals may be sent in respective pings in a multi-ping message.

FIG. 6G shows a seventh multimission survey including a first watercolumn mission and a second bathymetry mission 600G.

The first water column mission utilizes a relatively low frequency bandwith a CW or FM signal having a center frequency of about 100 kHz andrespective bandwidths of about 10 to 30 kHz and about 30 to 60 kHz.

The second bathymetry mission utilizes a relatively higher frequencyband with a CW or FM signal having a center frequency of about 400 kHzand respective bandwidths of about 20 to 60 kHz and about 30 to 60 kHz.These signals may be paralleled (as shown) in a single ping message.These signals may be sent in respective pings in a multi-ping message.Applicant notes the center frequencies of the signals mentioned inconnection with FIGS. 6A-E are examples. In various embodiments, thesecenter frequencies may vary in ranges of +/−5%, +/−10%, +/−25% and/or+/−50%. Applicant notes that the bandwidths of the signals mentioned inconnection with FIGS. 6A-E are examples. In various embodiments, thesebandwidths may vary in the ranges of +/−5%, +/−10%, +/−25% and/or+/−50%.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to those skilledin the art that various changes in the form and details can be madewithout departing from the spirit and scope of the invention. As such,the breadth and scope of the present invention should not be limited bythe above-described exemplary embodiments, but should be defined only inaccordance with the following claims and equivalents thereof.

What is claimed is:
 1. A sonar system comprising: a transceiver forconnection to a plurality of projectors in a projector array and aplurality of hydrophones in a hydrophone array where the arrays are notarranged in plural rows and plural columns; a transceiver transmitterand the projector array configured to project an elongated transmit beamthat simultaneously ensonifies an entire swath; a transceiver receiverand the hydrophone array configured to form elongated receive beams;wherein the system is configured to include a) consecutive transmissionsincluding a first transmission and a second transmission that form firstand second transmit beams where each of the transmit beams incorporatesat least first and second signals in first and second non-overlappingfrequency bands, b) the first transmit beam intersected by a first setof multiple receive beams and the second transmit beam intersected by asecond set of multiple receive beams, and c) the receive beams receivingechoes from scattering centers ensonified by the transmit beams.
 2. Thesonar system of claim 1 wherein: the first transmission is a first ping;the second transmission is a second ping; and, a ping cycle includes aping and reception of echoes from that ping.
 3. The sonar system ofclaim 2 further comprising: a third consecutive transmission or pingresulting in a third transmit beam conveying a third signal in a thirdfrequency band that does not overlap the first or the second frequencybands.
 4. The sonar system of claim 3 wherein the second frequency bandis higher in frequency than the first frequency band and the thirdfrequency band is higher in frequency than the second frequency band. 5.The sonar system of claim 3 further including: the first ping used inthe first ping cycle; the second ping used in the second ping cycle; thethird ping used in the third ping cycle; and, the second ping cycle isbetween the first and third ping cycles, the end of the first ping cycleabutting the beginning of the second ping cycle and the beginning of thethird ping cycle abutting the end of the second ping cycle.
 6. The sonarsystem of claim 3 further including: the first ping used in the firstping cycle; the second ping used in the second ping cycle; the thirdping used in the third ping cycle; and, the second ping cycle is betweenthe first and third ping cycles, the end of the first ping cyclesubstantially abutting the beginning of the second ping cycle and thebeginning of the third ping cycle substantially abutting the end of thesecond ping cycle.
 7. The sonar system of claim 3 further including: thefirst ping used in the first ping cycle; the second ping used in thesecond ping cycle; the third ping used in the third ping cycle; thesecond ping cycle is between the first and third ping cycles; t1 is atime period required to transmit the first ping; the first and secondping cycles separated by a time t12 of less than about 10 percent of thetime t1; and, the second and third ping cycles separated by a time t23of less than about 10 percent of the time t1.
 8. The sonar system ofclaim 3 further including: the first ping used in the first ping cycle;the second ping used in the second ping cycle; the third ping used inthe third ping cycle; the second ping cycle is between the first andthird ping cycles; t2 is a time period required to transmit a secondping; the first and second ping cycles separated by a time t12 of lessthan about 10 percent of the time t2; and, the second and third pingcycles separated by a time t23 of less than about 10 percent of the timet2.
 9. The sonar system of claim 3 further including: the first pingused in the first ping cycle; the second ping used in the second pingcycle; the third ping used in the third ping cycle; the second pingcycle is between the first and third ping cycles; t3 is a time periodrequired to transmit a second ping; the first and second ping cyclesseparated by a time t12 of less than about 10 percent of the time t3;and, the second and third ping cycles separated by a time t23 of lessthan about 10 percent of the time t3.
 10. The sonar system of claim 3further including: the first ping used in the first ping cycle; thesecond ping used in the second ping cycle; the third ping used in thethird ping cycle; the second ping cycle is between the first and thirdping cycles; t_(sd) is a time period required to transmit the pinghaving the shortest duration; the first and second ping cycles separatedby a time t12 of less than about 10 percent of the time t_(sd); and, thesecond and third ping cycles separated by a time t23 of less than about10 percent of the time t_(sd).
 11. The sonar system of claim 3, whereinthe first ping is spaced apart in time from the second ping and thesecond ping is spaced apart in time from the third ping.
 12. The sonarsystem of claim 3 further including: the first ping used in the firstping cycle; the second ping used in the second ping cycle; the thirdping used in the third ping cycle; the second ping cycle is between thefirst and third ping cycles; t1 is a time period required to transmitthe first ping; the first and second ping cycles separated by a time t12of about 10 percent or more of the time t1; and, the second and thirdping cycles separated by a time t23 of about 10 percent or more of thetime t1.
 13. The sonar system of claim 3 further including: the firstping used in the first ping cycle; the second ping used in the secondping cycle; the third ping used in the third ping cycle; the second pingcycle is between the first and third ping cycles; and, t2 is a timeperiod required to transmit a second ping; the first and second pingcycles separated by a time t12 of about 10 percent or more of the timet2; and, the second and third ping cycles separated by a time t23 ofabout 10 percent or more of the time t2.
 14. The sonar system of claim 3further including: the first ping used in the first ping cycle; thesecond ping used in the second ping cycle; the third ping used in thethird ping cycle; the second ping cycle is between the first and thirdping cycles; t3 is a time period required to transmit a second ping; thefirst and second ping cycles separated by a time t12 of about 10 percentor more of the time t3; and, the second and third ping cycles separatedby a time t23 of about 10 percent or more of the time t3.
 15. The sonarsystem of claim 3 further including: the first ping used in the firstping cycle; the second ping used in the second ping cycle; the thirdping used in the third ping cycle; the second ping cycle is between thefirst and third ping cycles; t_(sd) is a time period required totransmit the ping having the shortest duration; the first and secondping cycles separated by a time t12 of about 10 percent or more of thetime t_(sd); and, the second and third ping cycles separated by a timet23 of about 10 percent or more of the time t_(sd).
 16. The sonar systemof claim 1 wherein: the first transmission is transmission of a firstmessage; the second transmission is transmission of a second message;and, a message cycle includes a transmitted message and reception ofechoes from that message.
 17. The sonar system of claim 16 furtherincluding: a third consecutive transmission of a third message resultingin a third transmit beam conveying at least a third signal in a thirdfrequency band that does not overlap the first or the second frequencyband; the first, second, and third messages used in respective first,second, and third message cycles; and, the second message cycle isbetween the first and third cycles, the end of the first message cycleabutting the beginning of the second message cycle and the beginning ofthe third message cycle abutting the end of the second message cycle.18. The sonar system of claim 16 further comprising: the firsttransmission is transmission of a first signal; the second transmissionis transmission of a second signal; and, a third consecutivetransmission of a third message resulting in a third transmit beamconveying a third signal in a third frequency band that does not overlapthe first or the second frequency band.
 19. The sonar system of claim 18wherein the second frequency band is higher in frequency than the firstfrequency band, and the third frequency band is higher in frequency thanthe second frequency band.
 20. A sonar system for installation on awater going vehicle comprising: a multibeam echosounder including atransmitter, a receiver, projectors, and hydrophones; the projectors arenot arranged in rows and columns; the transmitter for transmitting aplurality of X signals in respective frequency bands that support Xmeasurement functions; the X signals driving each of the projectors andcausing each projector to emit sound waves; the sound waves focused onan elongated region such that scattering centers within the elongatedregion are ensonified; the receiver and hydrophones forming multiplereceive beams that intersect the elongated region at multiple locations;and, the receiver acquiring echo data from the scattering centers at theintersections; wherein frequency dependent characteristics of theensonified scattering centers evident in the echoes enable themeasurement functions.
 21. A sonar system for installation on a watergoing vehicle comprising: a multibeam echosounder including atransmitter, a receiver, projectors, and hydrophones; the projectors arenot arranged in rows and columns; the transmitter for transmitting Xsignals driving each projector and causing each projector to emit soundwaves; the sound waves focused on an elongated region such thatscattering centers within the elongated region are ensonified; thereceiver and hydrophones forming multiple receive beams that intersectthe elongated region at multiple locations; and, the receiver acquiringecho data from the scattering centers at the intersections; wherein echodata in each frequency band is processed such that at each frequency, afrequency dependent scattering center characteristic enables ameasurement function.
 22. A sonar method comprising the steps of: on awater going vehicle, providing a sonar including a transceiver, aplurality of transducers in a projector array, and a plurality oftransducers in a hydrophone array wherein the projectors in theprojector array are not arranged in rows and columns; configuring thetransceiver, projector array, and hydrophone array to form intersectingtransmit and receive beams; transmitting a first signal in a firstfrequency band; collecting echo data via first receive beams fromscattering centers ensonified by the first signal; transmitting a secondsignal in a second frequency band that does not overlap the firstfrequency band; collecting echo data via second receive beams fromscattering centers ensonified by the second signal; and, from the echodata resulting from the first and second signals, performing one or moremeasurement functions that indicate one or more frequency dependentcharacteristics of the ensonified scattering centers.